The invention relates to a volume quantity dispensing unit which can be used in an exhaust gas aftertreatment unit, having a pressure transducer, whereas the volume quantity dispensing of the volume quantity dispensing unit follows an electric signal and the volume quantity dispensing unit is calibrated. Furthermore, the invention relates to the disclosure of methods for setting or calibrating the pressure output signal volume quantity characteristic of a volume quantity dispensing unit which can be used in an exhaust gas aftertreatment unit.
In the sense of the present invention, the term “calibration” is used differently to the manner in which it is used in conventional metrology. In conventional metrology, the term “calibration” refers to the adaptation of a value to a given standard value and the attempt to minimise deviation between both values, the standard value and the calibrated value.
The present document refers to a volume quantity dispensing unit. There are known regulators which monitor and/or measure and/or regulate with the aid of the value not the volume quantity dispensing but rather the mass quantity. Because there is a direct physical dependency, for example via temperature, density and viscosity, between volume quantity and mass quantity, the two terms are used interchangeably.
The exhaust gases of modern combustion engines contain pollutants such as, for example, nitrogen oxides. In the field of exhaust gas aftertreatment in combustion engines, in particular in diesel engines run on a lean mixture, urea SCR (selective catalytic reduction) technology is currently the most efficient way to reduce nitrogen oxides. In a variation of urea SCR technology, a liquid urea/water solution is added to the flow of exhaust gas of the combustion engine at a suitable location before the SCR exhaust gas catalyst directly via a metering device comprising an injector device, for example a fine atomising nozzle. In another variation, a urea/water aerosol is added. In the SCR exhaust gas catalyst, the nitrogen oxides are then converted, as is known, into elemental nitrogen and water vapour by the ammonia cleaved from the urea/water solution or the urea/water aerosol (thermolysis and hydrolysis).